net: dsa: mv88e6xxx: implement port_vlan_dump

Remove the port_pvid_get and vlan_getnext functions in favor of a
simpler mv88e6xxx_port_vlan_dump function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>